1. Field of the Invention
This invention relates to materials for optical waveguides, and more particularly to waveguide materials having a relatively high index of refraction and/or a coefficient of thermal expansion which is nearly the same as the coefficient of thermal expansion of silicon.
2. Technical Background
Silicon oxynitride is a useful material for waveguides in planar photonic devices, as well as for barrier layers and dielectrics in displays and semiconductor devices. Silicon oxynitride is also of interest for use as a material for fabrication of photosensitive optical fiber. By varying the nitrogen/oxygen ratio, films with a wide range of refractive indices and thermal expansions can be produced. In particular, silicon oxynitride films with a high nitrogen content are of interest for planar waveguides in optical devices utilizing liquid crystals as an electro-optic material for total internal reflectance, in which a high refractive index (e.g., about 1.6 to about 1.8 for light at a wavelength of 1550 nm) planar waveguide is required.
Typically, silicon oxynitride is deposited by a chemical vapor deposition (CVD) technique, such as plasma enhanced chemical vapor deposition (PECVD) from the reaction of silane (SiH4) and ammonia (NH3). PECVD is favored for silicon oxynitride deposition because growth rates can be as high as 15 micron/hour. However, the reaction of silane and ammonia leads to the incorporation of large amounts of hydrogen (up to 20% for Si3N4) in the films. This has several undesirable effects. First, the incorporation of large amounts of hydrogen in the films results in high optical loss at 1550 nm. Another problem with incorporation of large amounts of hydrogen is that undesirable reduction-oxidation reactions occur between the hydrogen and surrounding materials. A further undesirable effect attributable to the incorporation of large amounts of hydrogen in a CVD deposited silicon oxynitride film is that such films cannot be as uniformly etched as films incorporating lower amounts of hydrogen.
When a large amount of hydrogen is present in a glass film formed by chemical vapor deposition, much of the hydrogen can usually be removed with heat treatment techniques. However, hydrogen is particularly difficult to thermally out-diffuse from a silicon oxynitride film formed by chemical vapor deposition because of the low permeability of hydrogen in the silicon oxynitride film. Unless careful and time-consuming procedures are followed, the films tend to blister and crack. The non-uniform dry-etching observed in high index silicon oxynitride films results from nanoscale (having a size of from about 1 nm to about 1 micron) structures that contain both porosity that entraps hydrogen, and dense highly strained regions. Thermal annealing to remove hydrogen, and relax and compact (densify) the. glass film is required to achieve uniform etch rates and precisely defined etched structures.
Another disadvantage with silicon oxynitride films deposited by CVD results from the large thermal expansion mismatch between silicon and silicon oxynitride films having an appropriate nitrogen content for achieving the high refractive index required for liquid crystal optical devices. This thermal mismatch leads to high film strains which cause birefringence in waveguides, substrate curvature, and can cause film cracking and/or delamination. Because of the disadvantages associated with incorporation of large amounts of hydrogen, and the large thermal expansion mismatch between silicon and silicon oxynitride films, the fabrication of planar photonic devices using high index silicon oxynitride derived from the reaction of silane and ammonia does not appear to be commercially viable.
Therefore, it would be highly desirable if a high refractive index waveguide material having a coefficient of thermal expansion which closely matches the coefficient of thermal expansion of silicon could be provided. It would also be highly desirable if a high refractive index waveguide material incorporating lower amounts of hydrogen and/or which could be more easily treated to remove incorporated hydrogen could be provided. Even more desirable, would be a high refractive index waveguide material which has both a coefficient of thermal expansion which closely matches the coefficient of thermal expansion of silicon, and which incorporates a relatively lower amount of hydrogen during chemical vapor deposition and/or allows easier removal of hydrogen incorporated during chemical vapor deposition.
The invention overcomes the problems inherent with high index silicon oxynitride films formed by chemical vapor deposition, and provides a commercially viable method of fabricating a high refractive index waveguide material. More specifically, the invention provides a germanium silicon oxynitride material having an inherently lower hydrogen content as deposited than silicon oxynitride; a higher hydrogen permeability than silicon oxynitride, which facilitates hydrogen removal; and a coefficient of thermal expansion which closely matches the coefficient of thermal expansion for silicon. These properties are extremely useful for fabricating optical devices based on total internal reflectance of liquid crystals.
In accordance with one aspect of the invention, a composition represented by the formula Si1xe2x88x92xGexO2(1xe2x88x92y)N1.33y is provided wherein x is from about 0.05 to about 0.6 and y is from about 0.14 to about 0.74. Such compositions exhibit a relatively high index of refraction, and a coefficient of thermal expansion which closely matches the coefficient of thermal expansion for silicon.
In accordance with another aspect of the invention, the germanium silicon oxynitride composition has an index of refraction of from about 1.6 to about 1.8 for light at a wavelength of 1550 nm.
In accordance with another aspect of the invention, the germanium silicon oxynitride composition has a coefficient of thermal expansion of from about 2.5xc3x9710xe2x88x926xc2x0 C.xe2x88x921 to about 5.0xc3x9710xe2x88x926xc2x0 C.xe2x88x921.
In another aspect of the invention, a germanium silicon oxynitride film is deposited on a silicon substrate. The germanium silicon oxynitride film has an index of refraction of from about 1.6 to about 1.8 for light at a wavelength of 1550 nm, and a coefficient of thermal expansion of from about 2.5xc3x9710xe2x88x926xc2x0 C.xe2x88x921 to about 5.0xc3x9710xe2x88x926xc2x0 C.xe2x88x921.
In accordance with a further aspect of the invention, a germanium silicon oxynitride film deposited on a silicon substrate is represented by the formula Si1xe2x88x92xGexO2(1xe2x88x92y)N1.33y, wherein x is from about 0.05 to about 0.6 and y is from about 0.14 to about 0.74.
In accordance with another aspect of the invention, a process for forming a layer of glass having a relatively high index of refraction and a coefficient of thermal expansion which closely matches that of silicon is provided. The process includes the steps of providing a substrate, and depositing on the substrate a layer of material represented by the formula Si1xe2x88x92xGexO2(1xe2x88x92y)N1.33y, wherein x is from about 0.05 to about 0.6 and y is from about 0.14 to about 0.74.